Si.sub.3 N.sub.4 has been known as a ceramic since 1857, but its structure was not determined with any degree of certainty until the 1950's. Useful objects have been fabricated from this ceramic by essentially two methods or arts: (1) hot pressing Si.sub.3 N.sub.4 powder mixed with pressing aids, or (2) reaction bonding silicon powder mixed with a catalyst by heating in nitrogen to be followed by densification. The two modes were combined U.S. Pat. No. 3,839,540, permitting pressing aids to be added to silicon prior to reaction bonding heating.
In the hot pressing art, pressing aid additives were deemed necessary because pure Si.sub.3 N.sub.4 is almost unsinterable, even under pressure. Typically, such aids have included the oxides of Mg, Y, Ce, Fe, Ca, Cr, Zr, Zn, Be, Al and selected rare earths (see U.S. Pat. Nos. 3,830,652; 4,304,576; 3,903,230; 3,839,540; 4,046,580; 4,234,343; 4,202,698; and 4,038,092). The pressing aids function as a means to flux the fusion or densification of the Si.sub.3 N.sub.4 powder during hot pressing by the formation of a liquid. Unfortunately, such liquid more readily forms a type of glassy or amorphous phase in the final product, which inhibits high temperature physical properties of the ceramic and reduces the life of a cutting tool made from this material. This is exemplified clearly by U.S. Pat. No. 3,839,540, which discloses a typical method of mixing silicon with a glass forming oxide (MgO), nitriding the mixture, and hot pressing at a high temperature of 1700.degree.-1750.degree. C. (see column 3, line 6 and column 4, line 30).
With this teaching, a variety of glass compounds in the nitrided body present an indefinite melting temperature range for the flux since each glass compound will have its own high melting range. As a result, the temperature for melting all of the glass species is very high, requiring 1700.degree.-1750.degree. C. to achieve, at best, a relatively low densification of 2.7-3.2 gm/cc. Under the conditions of the hot pressing art, the mixture does not form phases which crystallize upon cooling from hot pressing conditions. There is a need to modify the mixture prior to hot pressing to avoid a gross glassy phase.
There has been little recognition in the hot pressing art of the role played by the formation of a glassy as opposed to a crystalline intergranular phase with respect to tool wear. Tool wear consists of two components: the mechanical component and the chemical component. The mechanical component is a function of physical microstructure, for example, the grain size and the grain size distribution. The chemical component is a function of the chemical reaction of the intergranular phase as, for example, glassy or crystalline, or the chemical type of crystalline phase. Investigators of hot
detected the presence of various pressed Si.sub.3 N.sub.4 have crystalline phases (e.g., oxynitrides) without understanding their significance or the interrelationship of the crystalline phase to ceramic tool life and performance. Also, there has been little recognition of the role played by the retention of a small, controlled quantity of the glassy phase in protecting the crystalline phase, such as an oxynitride, against high temperature oxidation. In fact, the prior art has generally followed the axiom that adding oxygen to a powder system for forming a ceramic by nitriding will be detrimental and thus has taught in a direction away from that of this invention.
In the reaction bonding art, oxides of aluminum have been mixed with silicon powder to promote a sialon product as a result of nitriding, see U.S. Pat. No. 4,038,092 and the article by K. H. Jack and W. J. Wilson, "Ceramics Based on the Si-Al-O-N and Related Systems", Nature (London) Physical Science, 233 (80) 25-29 (1972). Oxides of yttrium have been added to silicon powder prior to nitriding to promote subsequent sintering (see U.S. Pat. No. 4,285,895), or added to silicon nitride formed by the reaction of Si, Al.sub.2 O.sub.3, and N.sub.2 for the purpose of facilitating sintering (see U.S. Pat. No. 4,184,884). However, users of Al.sub.2 O.sub.3 or Y.sub.2 O.sub.3 in such reaction bonding techniques again have failed to recognize the role that can be played by the formation of certain second phases, particularly silicon oxynitrides, derived from the additives as a displacement for the glass phase.
This lack of understanding is evident particularly in U.S. Pat. No. 4,285,895, wherein it is stated that a chemical analysis of the nitrided body exhibited 91% Si.sub.3 N.sub.4 and 9% Y.sub.2 O.sub.3 ; the phase composition was stated to be predominantly alpha silicon nitride with minor phases of beta silicon nitride along with a yttrium, silicon, oxygen, nitrogen phase (see column 6, lines 1-6). However, no distinction was made as to the type of phase, crystalline or glass, or even, if assumed crystalline, which particular oxynitride or complex oxide was present. The second phase most likely was a glass because the disclosure requires that sintering be carried out at a very high temperature, 1925.degree. C., one which is only necessary to decrease the viscosity of the glass to allow for sufficient mass transport or atomic mobility to effect densification. Most likely the glasses were formed because the densification aid (MgO or Y.sub.2 O.sub.3) was impregnated into the nitrided body and thus was reacted under high sintering temperature conditions (see column 4, lines 7-9). This patent failed to provide for the production of crystallites during nitriding and failed to recognize the role played by a controlled quantity of glassy phase that can be effective in protecting crystallites, if formed, against high temperature oxidation. The generation of the right crystallite, i.e., an oxynitride, encapsulated by a controlled glassy layer, can increase tool life by an increase in hardness and chemical stability and provide greater microstructure uniformity as demonstrated herein. U.S. Pat. No. 4,285,895 exhibited room temperature strength levels of 60,000-77,000 psi as opposed to 100,000-140,000 realized herein.
This disclosure is concerned with the additives that must be selected and used in reactive amounts and under conditions that result in the formation of desirable second phase crystallites, advantageously accompanied by a small glass envelope; of particular interest is the use of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3. The mere use of ingredients similar to that of this invention does not ensure the total formation of crystallites, but such goal is dependent upon proper ingredient proportioning and treatment prior to heating for densification as well as treatment during densification heating to form optimum second phase crystallites which are required in the final product to provide quality benefits for machining applications. The formation of the desired second phase crystallite will provide uniform melting of the second phase, will lower the hot pressing temperature, and reduce the hot pressing time. This will in turn result in a uniform microstructure with no visual "mottle-porosity" appearance.